1. Field of the Invention
The present invention relates to a data-transmission device, a data-reception device, a data-transmission-and-reception system, a method of controlling the data-transmission device, a method of controlling the data-reception device, a program, and a recording medium, and particularly relates to a technology appropriate for controlling a retransmission operation according to the line status and/or the packet importance.
2. Description of the Related Art
As communication systems have become more advanced in recent years, it has become a common practice to view video data requiring a relatively large data-communication band via a communication line such as the Internet. A system referred to as a transport protocol for real-time applications (RTP) has been commonly used, so as to perform the above-described video-data communications. The above-described system is particularly suited for transmitting video data requiring real-time characteristics, such as live video, over a long time period. The above-described RTP is a protocol defined as RFC 1889 and RFC 1890 by the Internet engineering task force (IETF), so as to transfer audio data, video data, etc. in real time.
Generally, real-time communications typified by the above-described RTP protocol are not always performed with high data reliability. However, a low-layer protocol belonging to a low level, such as a user datagram protocol/Internet protocol (UDP/IP) which is expected to be relatively simple and attain a high communication speed has been used. The RTP is especially designed with the above-described situation in mind. Although the use of a protocol such as the above-described UDP/IP allows for attaining favorable real-time characteristics, an error such as a packet loss easily occurs due to the property of the protocol.
If the above-described error occurs, there is a high possibility that a phenomenon in which part of video data is lost, etc. occurs. Therefore, various types of schemes have been devised. For example, an arrangement designed to limit a range that would be affected by the error at the encoding time has been incorporated into compressed and encoded video data. Further, making a loss of video data inconspicuous at the reproducing time, the loss being caused by an error that had occurred, has been actually performed. Further, in the light of communications, communication lines with reliability higher than ever have been proposed so that the error rate has been significantly reduced under predetermined conditions.
On the other hand, under circumstances where errors easily occur due to heavy communications, one of important tasks is to reduce the errors. In the past, technologies of controlling a packet-transmission method and performing communications with efficiency with consideration given to the state of a network where an error occurs and/or the importance of each packet have been proposed, as technologies to reduce an error such as a packet loss. As a known example of the above-described technologies, a technology disclosed in Japanese Patent Laid-Open No. 2004-248322 allows for decreasing the rate of packets with high importance when the rate of the packet losses is high and increasing the rate of the packets with high importance when the packet-loss rate is low at the time where the importance of packets is set on the transmission side. On the other hand, when the loss of a packet for which high importance is set is detected on the reception side, a request for retransmission of the packet is transmitted from the reception side to the transmission side.
According to the technology disclosed in Japanese Patent Laid-Open No. 2004-248322, however, in addition to the packet-loss detection, the reception side has to determine the importance of a lost packet and determine whether or not a retransmission request should be made when the packet loss occurs. Consequently, the processing load placed on the reception side increases.
Further, since an arrangement designed for determining the importance of the lost packet on the reception side has to be incorporated into each packet on the transmission side, the traffic increases by as much as the incorporation. Therefore, the disclosed technology is inadequate to perform communications with efficiency.
Still further, according the technology disclosed in Japanese Patent Laid-Open No. 2004-248322, the importance is measured in two levels, namely, high and low levels, which makes it difficult to perform sophisticated retransmission control. Therefore, in the eyes of the retransmission control, the disclosed technology is still inadequate to perform communications with efficiency.